It wasn't really a two year extension and it was before this season. It was really a one year extension where they spread the money he would have made this year over two years. I think it's the same deal they did with Demario Davis. So it was a pay cut in exchange for an extra year. I don't think it stops them from cutting him if they want to.
But, based on what Underhill and Tripp are saying, the decision on if they keep older players like Cam and Mathieu will depend on what they do with Carr. They can cut Carr but they would have to keep/convert the salary of all the older players to signing bonuses. Or, they can keep Carr and convert his salary to signing bonuses which would allow them to cut older players.
In the end, I don't think any of these decisions should be made now. They should hire a new HC and then let him decide how he wants to build his team.